The match_flag_type match_partial can be passed to the following algorithms: regex_match, regex_search, and regex_grep, and used with the iterator regex_iterator. When used it indicates that partial as well as full matches should be found. A partial match is one that matched one or more characters at the end of the text input, but did not match all of the regular expression (although it may have done so had more input been available). Partial matches are typically used when either validating data input (checking each character as it is entered on the keyboard), or when searching texts that are either too long to load into memory (or even into a memory mapped file), or are of indeterminate length (for example the source may be a socket or similar). Partial and full matches can be differentiated as shown in the following table (the variable M represents an instance of match_results as filled in by regex_match, regex_search or regex_grep):

Be aware that using partial matches can sometimes result in somewhat imperfect behavior:

The following example tests to see whether the text could be a valid credit card number, as the user presses a key, the character entered would be added to the string being built up, and passed to is_possible_card_number. If this returns true then the text could be a valid card number, so the user interface's OK button would be enabled. If it returns false, then this is not yet a valid card number, but could be with more input, so the user interface would disable the OK button. Finally, if the procedure throws an exception the input could never become a valid number, and the inputted character must be discarded, and a suitable error indication displayed to the user.

#include <string>
#include <iostream>
#include <boost/regex.hpp>

boost::regex e("(\\d{3,4})[- ]?(\\d{4})[- ]?(\\d{4})[- ]?(\\d{4})");

bool is_possible_card_number(const std::string& input)
{
   //
   // return false for partial match, true for full match, or throw for
   // impossible match based on what we have so far...
   boost::match_results<std::string::const_iterator> what;
   if(0 == boost::regex_match(input, what, e, boost::match_default | boost::match_partial))
   {
      // the input so far could not possibly be valid so reject it:
      throw std::runtime_error(
         "Invalid data entered - this could not possibly be a valid card number");
   }
   // OK so far so good, but have we finished?
   if(what[0].matched)
   {
      // excellent, we have a result:
      return true;
   }
   // what we have so far is only a partial match...
   return false;
}

In the following example, text input is taken from a stream containing an unknown amount of text; this example simply counts the number of html tags encountered in the stream. The text is loaded into a buffer and searched a part at a time, if a partial match was encountered, then the partial match gets searched a second time as the start of the next batch of text:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <boost/regex.hpp>

// match some kind of html tag:
boost::regex e("<[^>]*>");
// count how many:
unsigned int tags = 0;

void search(std::istream& is)
{
   // buffer we'll be searching in:
   char buf[4096];
   // saved position of end of partial match:
   const char* next_pos = buf + sizeof(buf);
   // flag to indicate whether there is more input to come:
   bool have_more = true;

   while(have_more)
   {
      // how much do we copy forward from last try:
      unsigned leftover = (buf + sizeof(buf)) - next_pos;
      // and how much is left to fill:
      unsigned size = next_pos - buf;
      // copy forward whatever we have left:
      std::memmove(buf, next_pos, leftover);
      // fill the rest from the stream:
      is.read(buf + leftover, size);
      unsigned read = is.gcount();
      // check to see if we've run out of text:
      have_more = read == size;
      // reset next_pos:
      next_pos = buf + sizeof(buf);
      // and then iterate:
      boost::cregex_iterator a(
         buf,
         buf + read + leftover,
         e,
         boost::match_default | boost::match_partial);
      boost::cregex_iterator b;

      while(a != b)
      {
         if((*a)[0].matched == false)
         {
            // Partial match, save position and break:
            next_pos = (*a)[0].first;
            break;
         }
         else
         {
            // full match:
            ++tags;
         }

         // move to next match:
         ++a;
      }
   }
}